Radio receiving apparatus for locating a plurality of target transmitters

ABSTRACT

Apparatus combining a UHF radio receiver with digital circuitry for processing and displaying signals indicating the location of one or more remotely located target radio transmitters, which apparatus includes a signal storing bearing indicator device operated in synchronism with a continuously rotating receiver antenna, and which apparatus also includes a refined AGC system and a target verifying correlation system for improving reliability of the information presented to an operator.

United States Patent [191 Bornhorst et a1.

[ RADIO RECEIVING APPARATUS FOR LOCATING A PLURALITY OF TARGET TRANSMITTERS [75] Inventors: Kenneth F. Bornhorst, Dayton; Jack L. Monroe, Tipp City; Alvin E. Culbertson, Dayton, all of Ohio; David E. Thomas, Columbia, S.C.; Donald E. Drake, Tipp City, Ohio [73] Assignee: The National Cash Register Company, Dayton, Ohio 22 Filed: Feb. 11, 1972 21 Appl. No.: 225,519

[52] U.S. Cl. 343/118, 343/112 PT [51] Int. Cl. G015 3/58 [58] Field of Search .i 343/118 [5 6] References Cited UNITED STATES PATENTS 9/1964 Green 343/118 X Oct. 16, 1973 2/1967 Distler et al. 343/118 X 6/1969 McEvoy 343/118 X Primary Examiner-Benjamin A. Borchelt Assistant Examiner-Richard E. Berger Attorney-J. T. Cavender et a1.

[5 7] ABSTRACT Apparatus combining a UHF radio receiver with digital circuitry for processing and displaying signals indicating the location of one or more remotely located target radio transmitters, which apparatus includes a signal storing bearing indicator device operated in synchronism with a continuously rotating receiver antenna, and which apparatus also includes a refined AGC system and a target verifying correlation system for improving reliability of the information presented to an operator.

13 Claims, 14 Drawing Figures PATENTEDUEI 16 I973 SHEET O70F 12 M M Ma:

PAIENIEDIICI I6 I973 3.766560 CONDUCTIVE NOTE GROUND PLANE *=STRAIGHT LINE DISTANCES BETWEEN INDICATED COMPONENTS.

OTHER DIMENSIONS ARE COAXIAL CABLE LENGTHS.

RADIO RECEIVING APPARATUS FOR LOCATING A PLURALITY OF TARGET TRANSMITTERS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention herein described was made in the course of or under a contract or a subcontract thereunder with the US. Air Force, Contract Number F3365 7- 67-C-l083-P00l, and pertains to an improved radio frequency homing receiver capable of processing signals from a plurality of targets and of displaying reliable indications of such targets to an operator in a concise and easily usable manner.

2. Description of the Prior Art Prior art radio frequency receiving apparatus "of the direction finding and homing type is primarily concerned with processing signals received from a single target transmitter together with providing an indication of the target to receiver distance, with much of such prior art apparatus accomplishing homing by'the use of signal null detection. The present invention overcomes the single target limitations and the null signal detec- 1 tion limitation while also providing improvements in the receiver AGC and target verifying systems. V

BRIEF SUMMARY or THE INVENTION The invention contemplates radio frequency receiver apparatus combining in several arrangements the functions of multiple target processing and display, signal peak detection, stable, electrically-controlled tuning, target verification through correlation, multiple AGC circuits for enhancing the target indicating and information communicating capability of a received signal and signal filtering for discriminating between expected target signals and noise signals.

The invention also contemplates a digital data displaying device for maintaining long duration displays of plural information bits in a. small easily discerned manner and for providing previously received signals for correlation with later received signals.

The invention also contemplates multiple automatic gain control (AGC) circuits for developing both a long term gated AGC signal and a fast reacting AGC signal.

BRIEF DESCRIPTION OF DRAWINGS FIGS. 1A, 1B, 1C, 1D, 1E and IF of the drawings, hereinafter referred to generally as FIG. 1 when combined in the manner as shown in FIG. 7, show a block diagram of a radio frequency homing receiver system made in accordance with the present invention;

FIGS. 2A and 2B show an exploded perspective view and a front view, respectively, of a bearing indicator which may be used with the system of FIG. 1;

FIG. 3 of the drawings shows an electrical block diagram of circuitry employed in mechanizing the bearing indicator shown in FIG. 2;

FIG. 4 shows an isometric view of a rotatable directive antenna used with the system of FIG. 1;

FIG. 5 shows an electrical schematic diagram of an automatic gain control circuit used in the system of FIG. 1;

FIGS. 6A and 6B combine to show an electrical schematic diagram of an automatic gain control system usable in the system of FIG. 1; and

FIG. 7 of the drawings, located on the sheet containing FIG. 1C, illustrates the manner in which FIGS. lA-lF fit together to form the FIG. 1 system.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following description relates to radio frequency receiving apparatus adapted to receiving signals from UHF emergency beacon transmitters commonly used by aircraft pilots as an aid to rescue and evacuation in hostile terrain, which apparatus is especially suitable for mounting in a helicopter or other small rescue aircraft.

Although the apparatus may be called an automatic direction finding system (ADF System), it may also be called a homing apparatus since it provides guidance of a rescue vehicle toward one of a plurality of target transmitters located at arbitrary and unknown geographic points; this function being in contrast to the normal ADF function, wherein receiving apparatus is used in navigation between points of known geographic coordinates and wheren the ADF receiver provides only single signal processing capability. The presently described apparatus provides processing and displaying capability for up to 24 different target transmitters on a total of four reception frequencies. Also, such apparatus is capable of extracting both bearing data and audio frequency modulation data from the signal received from a target transmitter, the audio information being either human voice modulation or tone modulation.

The apparatus described utilizes continuous constant velocity rotation of a directional antenna in contrast with the familiar Adcock ADF system which employs a stationary but movable array of directive antennas. The apparatus described also differs from the Adcock apparatus in its sensing of the peak or maximum amplitude point of a received target signal in lieu of sensing the null or point of minimum signal reception. This ability of the system to operate with a high average signal rather than a null signal has important implications where it is desired to receive voice or other intelligence information from the target transmitter in addition to the location indicating signal, and constitutes an important improvement over the Adcock and other ADF systems which obligate the receiver to constantly operate with a minimum of received signals from the target transmitter for the most accurate guidance.

The receiver system of the FIG. 1 makeup of the drawings consists of a radio frequency signal handling section which commences with antenna 20 (FIG. 1A) and passes radio frequency signals along a path which includes the double conversion superheterodyne circuitry involving mixer circuits 40 and 54, intermediate frequency amplifiers 60, 62, 64 and 66 (FIG. 1B) and terminates with an audio signal terminal 82 and a pulse signal output, pulse 106 (FIG. 1C). The ADF receiver also includes digital logic circuitry as shown in the bottom portion (FIGS. 1D, 1E and IF) of FIG. 1 for correlating pulse signals 106 with the rotational position of antenna 20 and displaying the results of this correlation to an operator in a visual pattern.

The receiver of the preferred embodiment is intended for operation in the ultra high frequency band between the frequencies of 200 and 400 MHz with a highly directive physically rotating antenna array. The frequency of operation is not critical to the nature of the invention described herein since other apparatus within the scope of the invention could be designed to operate in any radio frequency band. The 200 to 400 MHz frequency band selected for the preferred embodiment is assigned by international agreement for military, governmental and emergency uses. The receiver apparatus of FIG. 1 divides the 200 to 400 MHz frequency range into six discrete intervals each covering a frequency span of 30 to SO MHZ, the span of 30 to 50 MHz being conveniently covered by a tuned circuit network having the desired selectivity or the desired Q. The tuned circuit for each of these intervals is selected by digital control signals in the receiver.

The ADF receiver apparatus of FIG. 1 also provides for automatic switching between four selected reception frequencies with the four selected frequencies being selectable at will to a resolution of 50 KHz by electrical tuning switches accessible to the operator.

Although apparatus similar to that shown in FIG. 1 of the drawings could be fabricated to employ almost any band of radio frequencies from the low frequency band up to and including the microwave frequency band, the indicated 200 to 400 MHz range in the ultra high frequency (UHF) band offers the convenience of line of sight transmission, physically small and mechanically rotatable antennas, good antenna directional characteristics, and convenient light-weight small size portable transmitting apparatus that is easily adapted to the pilot rescue environment. If the apparatus of FIG. 1 were employed at low frequencies an array of ferrite core antennas could be substituted for the physically rotating antenna shown in FIG. 1. At higher frequencies a rotating waveguide horn or electronically scanned stationary antenna could be employed with the FIG. 1 apparatus. a

The circuitry shown in FIG. 1 is descriptive of signal flow between processing portions of the ADF system, and while there is omitted such formalities as the receiver power supply, division of the system into mounting space dictated packages, and the actual wiring connections between signal processing circuits, it is believed that persons skilled in the art of ADF receiver design can provide these formal details from the prior art without difficulty.

In its simplest form, operation of the circuitry shown in'FIG. 1 consists of generating in the radio frequency circuitry of the receiver an electrical pulse indicating the reception of signal from a remote transmitter, this electrical pulse being the target data pulse 106 shown in FIG. 1. Once this pulse has been generated, the remaining portion of the FIG. 1 circuitry is used to relate the time occurrence of the target data pulse 106 to the antenna rotation position and to display this relation in a manner easily comprehended by an aircraft pilot. The relation of target data pulses to antenna position is achieved by loading target data pulses into a 180 bit shift register which shifts data pulses between bit positions each time the antenna rotation position has changed by 2, that is, rotation of the directional antenna is divided into 180 increments of 2 each with the target information located in each 2 rotation increment being resolved by the system and displayed to the operator. In its simplest form, the ADF system of FIG. 1 consists of a serial digital memory clocked in synchronism with rotation of the directional antenna array, a radio frequency receiver circuit, and an operator display apparatus.

The system of FIG. 1 also provides several capabilities which increase its overall usefulness, including electrical tuning of the receiver by either manual or sequential automatic control, double conversion superheterodyne radio frequency circuitry for better image signal rejection, provision for extracting an audio frequency signal in addition to a target data pulse from the received radio frequency signal, an elaborate automatic gain control network permitting operation over a wide range of signal strengths, a signal peak detecting circuit cooperative with the directive antenna array in order that target signals may be detected by their maximum received signal in lieu of the conventional minimal or null signal detection, a digital signal correlation apparatus for assuring the validity of displayed target indications, and a system of operator alerting lamps for indicating the presence of received but not displayed signal. The FIG. 1 system also includes special electrical filtering to remove the effects of aircraft propeller modulation and other interference from the received radio frequency signal.

Several portions of FIG. 1, including the directional antenna array 20, the operator display apparatus 173, the AGC system 72, and the low pass filter 84 are described in detail following the overall system description of the FIG. 1 apparatus. Considering such FIG. 1 apparatus, it is sufficient to note that the antenna 20 is a directive array of elements having approximately a 60 beam width, and is physically rotated by a drive rotor and gear box 26 to scan in all possible azimuth directions around the receiving apparatus.

As shown (FIG. 1A), digital signals indicating rotational position of the antenna 20 are generated by magnetic pickup heads 28 and 30 which engage rotating gears 24 and 25 mountedon an antenna shaft driven by the motor gear box combination 26. The signals generated by the magnetic pickup head 28 occur once for each rotation of the antenna 20 at approximately the l displacement position of the antenna while signals generated by the magnetic pickup head 30 occur every 2 of antenna rotation with the 1 pulse occurring substantially one-half way between the 0 and 2 pulses.

Radio frequency electrical signals representing the propagated waves received by antenna 20 are coupled through a rotating electrical coupling 22 into the receiving apparatus. The radio frequency signals are first applied to an RF pre-amplifier assembly which is located in the package 37 containing the antenna 20, the motor and gear box unit 26; signal transmission between the RF pre-amplifier assembly and the rotating electrical coupling 22 being accomplished with coaxial cable at some low impedance level such as 50 ohms. The radio frequency portion of the receiver apparatus of FIG. 1 consists of the RF pre-amplifier assembly, a high pass filter network 38, two double balanced diode mixers 40 and 54, an electrically tunable local oscillator 44, a crystal controlled load oscillator 56, a first intermediate frequency amplifier 52 located between the two mixer stages (all as shown in FIG. 1A), a crystal filter circuit 58, and broad band intermediate frequency amplifiers 60, 62, 64 and 66 which are connected to detector circuits 68, 74, 76, respectively, to provide audio and pulse output signals (FIG. 1B). The gain of several amplifier stages in the radio frequency portion of the receiver is controlled by a slow AGC signal generator 72, while gain of the audio frequency intermediate amplifier is controlled by a fast AGC amplifier 70.

During operation of the radio frequency portion of the FIG. 1 system, the RF signal is coupled through the rotating electrical coupling 22 from the antenna 20 to one of the six band filters 32 and a corresponding tuned RF amplifier 34 before passing through the broad band amplifier 36 into the high pass filter 38. Such RF signal from the broad band amplifier 36 is coupled by a low impedance coaxial cable into the high pass filter 38. The AGC signals feeding amplifier 33 in the RF preamplifier assembly are supplied in differential form as on AGC and AGC reference signals in order to minimize the amount of power supply and other noise impressed on the AGC signal applied to the tuned RF amplifiers. The use of a differential amplifier at 33 has been found to provide common mode noise signal rejection in the order of 30 db in the 400 Hz frequency band used for aircraft power supply. The non-linear gain control network 35 may be composed of some series/parallel combination of linear and nonlinear elements such as semiconductor junctions and resistors; the purpose of the nonlinear gain control network being to provide the exponential correlation needed between the AGC signal and the gain control function of the RF pre-amplifier circuit.

While several techniques are known in the prior art for introducing an AGC signal into an RF amplifier, in the present circuitry it was found convenient to employ a PIN diode connected in series with the radio frequency signal path to perform both gain control and selecting one of the six band filters shown at 32; the PIN diode being held reversed biased to block signal entry to a band filter and being held forward biased with a varying amount of forward current to select the conductance or gain of the diode as a signal transmitting element. In the FIG. 1 system it was found desirable to employ one PIN diode at the input of each of the band filters 32 and to vary the forward current in this diode as a function of the automatic gain control voltage and to employ another PIN diode at the output of each of the tuned RF amplifiers 34 and to switch the latter of these diodes between the conducting and nonconducting state by way of digital signals received from the band enable circuitry 39.

In the preferred embodiment of the invention, it was found desirable to incorporate about db of signal power gain into the RF pre-amplifier assembly. It was also found desirable to bias the tuned RF amplifiers 34 at an operating point providing best noise figure rather than maximum gain. With these operating conditions in the receiver front end, a sensitivity down to l microvolt at 30 percent modulation is achieved. The system is also capable of operating with up to 100,000 microvolts of input signal by way of the 60 db of automatic gain control provided, and likewise is capable of operating without damage when signals as large as 500,000 microvolts or )6 volt are applied.

Signals from the RF pre-amplifier assembly are applied to the high pass filter 38 which has an attenuation near 33 db at a frequency of 50 MHz and an attenuation of 3 db at 175 MHz. The high pass filter 38 is incorporated into the system in order that the receiver offers sufficient rejection to any 50 megacycle interference signal appearing at the antenna. Without the high pass filter 38, it was found that a 50 MHz interference signal could pass through the band filters and the double balanced mixer 40 into the tuned amplifier 52 with undesirable case.

After transmission through the high pass filter 38, an incoming RF signal is applied to the double balanced diode mixer circuit 40 where the incoming RF signal and signal from the electrically tunable local oscillator 44 are mixed to provide signals at the first intermediate frequency of MHz. Although the prior art in radio receiver design describes several circuits for mixing radio frequency signals, the double balanced diode mixer wherein a semiconductor diode bridge has a first center tapped coil connected across one axis of the bridge and a second center tapped coil connected across the other axis of the bridge, wherein the radio frequency input signal is applied at the first coil and the intermediate frequency output is taken from the second coil, and wherein the local oscillator signal is applied between the two coil center taps has been found to offer desirable economical mixing performance in a small package. The particular double balanced diode mixer employed in the preferred embodiment of the present invention provides a conversion loss of between 7 and 9 db with a noise figure in the 7 to 9 db range.

The tunable local oscillator in a superheterodyne receiver may take any one of several electrical forms including an oscillator tuned by mechanically varying the inductance or the capacitance elements of a resonant L-C circuit as is the arrangement found in most household AM and FM receivers, or a form of oscillator involving the use of plural quartz crystals in an arrangement permitting mechanical or electrical selection between different crystal units this technique being especially popular in military and limited frequency communications equipment. Although any of the known local oscillator tuning schemes could be used in an ADF receiver built according to the present invention, the environment of the present receiver calling for signal reception anywhere within a fairly wide range of radio frequencies makes the frequency synthesizer form of local oscillator practical. In a frequency synthesizer local oscillator, the oscillating frequency is altered by changing the characteristics of the digital feedback network coupling output and input of a phase locked oscillator loop. The phase locked loop as used in the frequency synthesizer is described in detail in many prior art publications, such as the article entitled, The Monolithic Phased Locked Loop A Versatile Building Block, by Allen B. Grebene, of Signatics Corportion, and appearing in the March 1971 issue of the magazine Spectrum published by the Institute of Electrical and Electronic Engineers. An integrated circuit phase locked loop is commercially available on the market and frequency synthesizer local oscillators have been employed in communication equipment of the Citizens Band two-way radio type for some time.

In the circuitry of FIG. 1, the output of the electrically tunable local oscillator (frequency synthesizer) is coupled through a broad band amplifier 42 into the double balanced diode mixer circuit 40. The broad band amplifier 42 provides isolation between the double balanced mixer and the frequency synthesizer local oscillator circuit. The output of the mixer circuit 40 is coupled into a 50 MHz double tuned amplifier 52 which may be designated as the first of several intermediate frequency amplifiers. The output of amplifier 52 is coupled into the second double balanced diodemixer circuit 54 along with the output of the second local oscillator 56 which is operated at a frequency of 60.7 MHz and is frequency stabilized by a quartz crystal. The employment of two mixer stages and two local oscillators, that is, the use of a double conversion front end in the FIG. 1 system was found desirable in reducing the image response of the receiver. The second double balanced diode mixer 54 can be identical with the first double balanced diode mixer 40.

Broad band Class A intermediate frequency amplifiers which are capable of being gain controlled over a large dynamic range are used in the FIG. 1 circuitry. With borad band intermediate frequency amplifiers, the needed frequency selectivity is vested in a crystal filter circuit that precedes the Class A amplifier circuit, this arrangement being shown at 58, 60 and 62 (FIG. 18) where the signal from the double balanced diode mixer 54 is fed through the quattz crystal filter 58 into the Class A amplifier circuits 60 and 62. A quartz crystal filter having 3 db half power points of 35 KHz that is, a filter having a center frequency of 10.7 MHZ and a total 3 db band pass of 35 KHz centered on the 10.7 MHz IF frequency, is quite satisfactory. Such a crystal filter also has a 60 db band width of 100 KHZ or less with a 6 db maximum insertion loss. The fixed gain broad band Class A intermediate frequency amplifier 60 is vested with a signal power gain of 30 db while i the variable gain broad band Class A IF amplifier 62 is vested with a gain controllable between --10 db and +30 db depending upon the applied AGC signal.

Signal at the output of variable gain amplifier 62 is a 10.7 MHz radio frequency carrier which may be modulated by an audio signal, a pulse signal and a 10 Hz sinusoid resulting from rotation of the directional antenna at a 600 RPM rate. The signal from variable gain amplifier 62 is applied to both variable gain amplifier 64 and variable gain amplifier 66, the amplifier 64 providing signal to the audio frequency envelope detector 68, while the amplifier 66 provides output signal for the binary or pulse circuitry of the system and signal for the automatic gain control system. The variable gain amplifier 64 is again a Class A amplifier capable of providing gain in the range of-lO db to +20 db according to the amount of gain controlling signal supplied by the fast AGC amplifier 70. The fast AGC amplifier 70 provides an output signal between 4 volts and 10 volts according to the magnitude of the signal received from the output of envelope detector 68, such AGC amplifier 70 having the characteristics of both rapid rise and rapid fall of its output signal in response to the output of envelope detector 68. The primary function of the fast AGC amplifier 70 is to remove a large portion of the 10 Hz modulation imposed by the rotating directional antenna 20 from the audio signal component of the RF signal re ceived from the variable gain amplifier 62. The overall effectiveness of the combined fast AGC amplifier 70 and the slow AGC amplifier 72 may be measured in the fact that signal at the output of envelope detector 68 does not fall outside the range of 100 to 200 millivolts even though the input signal at antenna 20 varies from 1 to 100,000 microvolts.

The 100 to 200 millivolt audio signal at the output of envelope detector 68 is applied to the narrow band audio amplifier 78 where signals suitable for driving an aircraft intercommunication system are developed, these signals appearing at terminal 82 (FIG. 1B). The narrow band audio frequency amplifier 78 is provided with a manually adjustable loudness control 80. The band pass of the audio frequency amplifier 78 is limited to the range of 500 Hz to 3,000 Hz at the 3 db down points, with the roll off below 500 Hz being 24 db per octave and the roll off above 3,000 I-Iz being 6 db per octave in order that signal modulation components outside the voice communications range be removed and not clutter the audio frequency signal.

Signal from the variable gain amplifier 62 is also applied to the variable gain amplifier 66 and thence to the two envelope detectors 74 and 76, which remove the radio frequency carrier and provide output signals representing the modulation component of the RF signal and the peak magnitude of the RF signal respectively. Signal from the envelope detector 74 is applied to the low pass filter network 84 and thence to the decision threshold network 86. The low pass filter network 84 is incorporated into the system primarily to remove from the signal the component of modulation resulting from rotation of propellers on some aircraft used to house the ADF system. Without the low pass filter 84 included in the circuitry, it was found that signals originating from a point behind the aircraft were processed with greater stability than were signals arriving from the frontalside of the aircraft where modulation by the propellers was encountered. The low pass filter network 84 has a pass band between the frequencies of 10 Hz and 65 Hz and an attenuation near 60 db at frequencies of Hz and above.

Signals originating in the commonly used frequency sweeping chopper modulated emergency rescue beacon transmitter contain a modulation component varying between about 240 and 3,000 Hz at a rate near to 3 Hz, this modulation component providing a distinctive woop-woop sound that is easly identified in rescue operations. Since such woop-woop modulation component would unnecessarily complicate the signal peak detecting function, the low pass filter 84 removes this modulation from the signal and provide an output indicating merely the presence or absence of a radio frequency signal. Signal applied to the low pass filter 84 is in the order of 1% volts peak-to-peak, with a 6 db variation resulting from db input changes at antenna 30.

Signal originating in envelope detector 76 represents the peak amplitude of the radio frequency signal and is applied to the AGC signal generator and amplifier circuit 72. As shown in detail in a later part of this specification, the circuitry of the AGC signal generator and amplifier 72 includes a capacitor memory element havlng a long decay time constant near 5 seconds in duration. The circuitry of the AGC signal generator and amplifier 72 provides what is named a fast attack slow release AGC signal, i.e., a AGC signal capable of quickly following magnitude increases in the 10.7 MHz pulses received from envelope detector 76, but falling from this peak amplitude at a very slow rate. At selected times in the FIG. 1 system operation it is desirable to quickly remove the AGC signal that has been developed. This removal is accomplished by a signal supplied on line 73 which originates in decoding circuitry (FIG. 1D) for the antenna revolutions counter.

The analog signal indicating presence or absence of a target radio frequency energy source as provided by the low pass filter 84 is applied to a threshold gate circuit 86 which removes low amplitude analog signals and transmits those analog signals which equal or exceed a preestablished threshold magnitude. The magnitude of signal required from the low pass filter 84 in order that an analog signal be passed through the threshold gate 86 is determined by the setting of sensitivity control 88, which is made accessible to the operator or alternately can be made a maintenance adjustment. Although the directional antenna 20 employed in the ADF system has its major response located in a single frontal lobe, the antenna inherently contains one or more minor response lobes directed to the sides and rear of the main response lobe. In the situation where a signal is received via the frontal lobe from a distant transmitter and from a nearby transmitter via the minor lobes it was found possible for the system to become confused through processing both the frontal and rearward lobe responses. For this reason threshold gate 86 was incorporated and greatly reduces the possibility of confusion by eliminating a large portion of the signals entering via minor antenna response lobes on the basis of their lower amplitude.

The presence of a signal from a target transmitter has been found to be reliably sensed in the present ADF system environment by detecting the peak of the approximately sinusoidal wave at the output of low pass filter '84, that is, by detecting the point of inflection or the point of zero slope in the low pass filter output. Detection of the peak of this wave is accomplished with the differentiator circuit 100 and the zero crossing amplifier 102 (FIG. 1C). These circuits perform the electrical equivalent of mathematically differentiating the low pass filter output signal in order that the peak of the wave from the filter 84 be marked by a change in signal polarity at the output of differentiator circuit 100', this change of signal polarity or crossing of zero voltage beingsensed and amplified by the circuit 102 which provides an output pulse whenever the polarity of its input circuit changes. The combination of differentiator circuit 100 and zero crossingdetector 102 may be embodied in any one of several ways which are known in the art, the operational amplifier embodiment of such circuits as shown in FIG. 6A of the drawings has a large dynamic operating range (near db) and can therefore accept a much greater range of input signals than would a simple differentiator circuit based upon driving a switching transistor into and out of the conducting state.

The output of zero cross detector circuit 102 is applied to the pulse shaping circuit 104 where an output pulse of uniform time duration and uniform rise and fall times is generated. The output of the pulse shaping circuit 104 is given the name target data pulse or TD1 pulse and is identified with the number 106 (FIG. 1C). The pulse shaping circuit 104 may consist of a monostable multivibrator triggered by the signals generated in the zero cross detector 102. The target data pulse or TD1 pulse 106 represents the output of the radio frequency portion of the automatic direction finding system. The TD1 pulse appears whenever the rotating antenna 20 locates a remote transmitter and scans through the point of maximum alignment with the remote transmitter, that is the TD1 pulse occurs just following the point of maximum signal reception from the remote transmitter.

The circuitry in the lower half of FIG. 1 (1D, 1E and IF) displays an indication of the antenna 20 rotational position at the instant a TD1 pulse occurs, and also displays an indication of the antennas rotational position for each TD1 pulse generated by the system. Such circuitry in the lower half of FIG. 1 also provides storage for the signal indicating TD1 pulse location within a rotation of antenna 20; incorporation of which storage capability enables the system to display a steady and continuous indication of a remote transmitter location and also permits the system to require in one mode of operation that signals be received from the remote transmitter in two successive antenna rotations at the same, or nearly the same, location before a display indication is made.

The amplifiers 108 and 110 (FIG. ID) are located physically in the package containing the drive motorgear box assembly 26 and the antenna 30, and are located several feet from the main portion of the ADF system. The amplifiers 108 and 110 amplify and sharpen the waveform of the rounded bipolar pulses originating in the magnetic pick-up heads 28 and 30 and convert these signals into a differential voltage suitable for transmission over the several feet of balanced twisted pair of conductors connecting the amplifiers 108 and 110 with the circuitry of blocks 112 and 1 14. The circuitry of the blocks 112 and 114 includes a balanced differential line receiver, a noise rejecting threshold circuit and also circuitry for converting signals from amplifiers 108 and 110 into digital pulses of uniform amplitude.

In a radio frequency receiver system which employs both a scanning antenna and an automatic gain control apparatus, some housekeeping rules are needed regarding the manner in which the automatic gain control apparatus responds to signals of different strength received at different times in the antennas scanning pattern; for instance, if the antenna encounters a very strong signal located at the zero degree scanning position, and a very weak signal located at the scanning position, it is possible for the housekeeping rules to dictate that the automatic gain control system first accommodate the receiver to the very strong zero degree signal and then immediately re-accommodate the receiver, by an increase of gain, to the very weak 90 signal. In the presently described ADF system, it was found that attempting multiple compensation of the receiver gain within a single antenna revolution leads to erratic information display requiring an undue amount of operator interpretation. A particularly troublesome form of this erratic response is found in the situation where the automatic gain control system has accommodated the receiver to a very weak signal at one azimuth position whereupon the antenna encounters a very strong signal at some successive azimuth position. Since the receiver system is adjusted for a high gain condition by the very weak signal, the encountering of a very strong signal will produce receiver overloading and generation of very long duration target pulses which will be displayed as a target located over a very wide band of azimuth positions. In the ADF system of FIG. 1, an elementary set of housekeeping rules for the AGC system is shown mechanized, these rules involving simply the requirement that the AGC signal generator and amplifier 72 provide an AGC signal corresponding to the strongest signal encountered by the an tenna 20 during preliminary revolutions which precede the actual receiving and processing of radio frequency signals. With these rules, part of the antenna revolutions in a sequence are used only for the purpose of adjusting the AGC system, with the remaining portion being used for the reception of target signals. Since the operator of the FIG. 1 system is provided with frequency selecting controls, it is possible for him to exclude, from even the AGC system of the receiver, signals which are located on any particular frequency even though these signals be very strong in comparison with an elected signal that is, reception of a weak signal on one frequency need not be jeopardized by the occurrence of a very strong signal on some other receiver frequency.

The ADF system shown in FIG. 1 is provided with two different modes of operation. One of these modes, which is called the single frequency mode, involves the receiver being tuned to a single reception frequency. This frequency, for instance, might be the internationally assigned distress frequency of 243 MHz. In the other mode of operation, called the all frequency mode, the system of FIG. 1 sequentially switches the receiver tuning between two, three or four different frequencies, each selectable by the operator. The single frequency mode of operation is intended for use in tracking or traveling toward one particular target transmitter while the all frequency mode is intended for searching a geographic area to find the approximate location of each one of several transmitting targets. In either the single frequency or the all frequency mode of operation, the principle of using preliminary antenna revolutions for the derivation of a suitable AGC signal and later revolutions for the actual reception of target data signals is adhered to.

In the all frequency mode of operation, the time required for the electrically tunable local oscillator (frequency synthesizer 44) to switch between frequencies and become stabilized at a new frequency has been found to be on the order of 20 milliseconds, since the antenna 20 requires approximately 100 milliseconds for a complete revolution it was found necessary to devote two preliminary revolutions of the antenna to the establishment of an AGC signal in order that at least one complete rotation occur after the receiver is actually tuned to a reception frequency.

The antenna revolutions counter 115, and its decoding logic 120 in FIG. 1D, control the receiver frequency changes, discharge times for the automatic gain control signal generator amplifier 72, the routing of target data pulses 106 into the display, memory and lamp circuits 172 and 174, and the correlation of presently received data with previously received data. The antenna revolutions counter 1 15 is a conventional four bit ripple down counter capable of 16 different output states, four of these states being devoted to signal processing in each of the four possible reception frequencies when the receiver is operated in the all frequency mode. In all frequency mode operation, the first four counts of the antenna revolutions counter 115 are devoted to signal reception on one frequency with part of the first one of these four counts being devoted to both achieving lock-on of the frequency synthesizer circuit 44 and to signal sampling for the AGC generator and amplifier 72, and with the second of the four counts being devoted entirely to signal sampling by the AGC signal generator and amplifier circuit 72 and with the third and fourth of these four counts being devoted to actual collection of target data signals. Commencing with the fifth count of the antenna revolutions counter 1 15, the receiver will be tuned to a new reception frequency wherein the four count sequence is repeated, in a similar manner commencing with the ninth and twelfth counts of the antenna revolutions counter, signals in the third and fourth reception frequency are processed, each for four counts of the counter 115.

The antenna revolutions counter 115 changes states once for each revolution of the antenna with the changes occurring upon receipt of the l antenna pulse from circuit 112 (FIG. 1D). In the all frequency mode of operation the antenna revolutions counter identifies sixteen consecutive revolutions of the antenna with each set of four of these revolutions being devoted to reception of signals in a different frequency band. In the single frequency mode of operation, the antenna revolutions counter 115 identifies only four antenna revolutions and the receiver is maintained in a single reception frequency so that, at the completion of our antenna revolutions, a new set of four revolutions is commenced without changing the receiver frequency. The digital processing for the single frequency mode as shown in FIG. 1 incorporates the signal correlating circuit requiring a signal be located in thesame or nearly the same position during two consecutive rotations of the antenna before display to the operator is made. Correlation is found to reduce the jitter or instability in target information displayed to the operator. In the FIG. 1 embodiment of the system, correlation capability is included only for the single frequency mode of op eration because of the larger amount of digital memory required for correlating signals received during the sixteen antenna revolutions involved in the all mode of operation.

Looking to the FIG. 1 drawing makeup, the shift register 168 (FIG. 1E) may be fabricated with flip flops or magnetic devices or any of the other embodiments for shift register circuitry known to persons skilled in the art; with flip flops having been used in the preferred embodiment of the invention. The essential property of the shift register is that data be loaded in serial form into one end of the register and be transferred serially from bit to bit within the register in coincidence with a clock or shift pulse. In the case of shift register 168, data is loaded serially into same from the OR gate 160 while shift pulses are applied through the amplifier 166. Shift register 168 also provides an output signal which represents at any given time the input signal that was introduced 180 shift pulses earlier, that is, a particular bit of data from the OR gate 160 when introduced into the shift register 168 and shifted by l shift pulses will reappear at a later time on the output line 159. The shift register 168 is also provided with a clear signal which passes through amplifier 164 and has the capability of resetting all bits of the memory into the zero state.

The first two bits of the antenna revolutions counter 115, identified with the number 116 (FIG. 1D), count antenna revolutions in both operating modes of the system, with the output signals from these two bits identifying revolutions l, 2, 3, and 4 of the antenna 20 in any operating mode. The switch 117, which is closed in the single frequency operating mode, disables the last two bits 118 of the counter by holding the memory elements comprising these two bits in the true condition. With the two highest order bits of the counter held in the true condition, the logic circuitry of the system will perform any fifth, ninth, or thirteenth revolution function during revolution 1 of the antenna and will perform any second, sixth, tenth, or fourteenth revolution function during revolution 2 thereof.

The 16 different states which the four bit antenna revolutions counter 115 may attain are decoded by the electrical circuitry as shown in the block 120; for instance, the signal applied to AND gate 122 from the antenna revolutions counter decoding 120 is driven to the true state during any of the revolutions 3, 4, 7, 8, ll, l2, or 16.

The TDl pulse 106 representing target data reaches the target data latch 142 (FIG. 1E) and the 180 bit shift register 168 via one of two signal paths depending upon whether the system is operating in the all frequency mode without data correlation or in the single frequency mode with data correlation. In the all frequency mode the TDl pulse is passed through AND gate 134 and OR gate 140 to reach the target data latch 142. Since the first memory elements of the shift register can receive target data only in the presence of a shift pulse, target data found within a 2 segment of antenna rotation is stored prior to a shift pulse in the target data latch 142. Target data from the target data latch 142 is coupled through the OR gate 160 into the first memory element of the shift register 168. As indicated by the signals coupled to AND gate 134, transmission of the TD1 pulse through the AND gate 134 requires that the antenna revolutions counter 115 be in a state indicating one of revolutions 3, 7, 11 or 15; that is, during each four revolution sequence of the antenna in the all frequency mode, a single antenna revolution, being the third revolution, is devoted to filling of the 180 bit shift register 168 and data collected in the fourth antenna revolution is not used since target correlation is not performed in the all frequency mode.

When the system is in the single frequency mode of operation, that is, operating in the manner intended for tracking of a single target transmitter, the TDl pulses enter the shift register 168 by a path different from that used for the all frequency mode. In the single frequency mode, the switch 131 is closed and the path for TDl pulses involves AND gate 133, one shotmultivibrator 136, AND gate 138, OR gate 140, target data latch 142, OR gate 160 and the shift register 168. In such mode, target data pulses which were loaded into the shift register 168 during the third antenna revolution emerge from the shift register on the signal line 159 at the same time that similar fourth revolution target pulses are being received in the form of TDI pulses; however, since the shift register 168 stores target data for precisely one revolution of the antenna, the line 159 output of shift register 168 is third revolution target data and is coincident with the real time target data of the fourth antenna revolution.

Since in a practical environment, a given target may appear in slightly different rotational positions in the third and fourth antenna revolution, it is found desirable to include circuitry in the FIG. 1 system for electronically stretching the duration of pulses representing both third revolution targets and fourth revolution targets in order that such third and fourth revolution pulses be assured of simultaneous existence and the ability to enable AND gate 138. In the FIG. 1D circuitry, the third revolution target data which has been stored by shift register 168 is elongated by the one-shot multivibrator 144 which has a time period in the order of 1.8 milliseconds, i.e., target pulses from the I80 bit shift register 168 normally having a time duration near 0.55 millisecond are stretched into a pulse of 1.8 milliseconds duration by the one-shot multivibrator 144. In a similar manner, the TD] pulse 106 is stretched by the one-shot multivibrator 136 into a pulse having time duration near 0.7 millisecond. When the elongated third revolution target pulse and the elongated fourth revolution target pulse overlap in revolutions 4, 8, l2 and 16, the conditions for AND gate 138 to pass signal are met and a target pulse, now a correlated target pulse, is

passed between AND gate 138 and OR gate 140 and thence into target data latch 142, OR gate and shift register 168.

The time duration of the one-shot multivibrators 144 and 136 is selected with a view toward the performance desired from the FIG. 1 system, the consideration in this selection being that the likelihood of coincidence presently received or new data in the one-shot multivibrator 136, it is possible to'realize correlation regardless of whether the fourth revolution pulse occurs slightly before or slightly after the third revolution pulse.

In the single frequency mode of operation, the correlation system provides a self clearing characteristic for shift register 168, i.e., it being not necessary to clear old third revolution target pulses from the shift register by use of the clear driver 164 because third revolution target pulses that are not substantiated or correlated by the presence of a similar target in the fourth revolution are automatically removed from storage by the lack of third, second, or fourth revolution correlation. Any third revolution target represented by a pulse from the one-shot multivibrator 144 which is not coincident with a fourth revolution pulse coming from the one-shot multivibrator 136 does not pass through the AND gate 138 and is therefore not replaced in storage in the shift register 168. As shown by that input of AND gate 138 originating in antenna revolutions counter decoding logic 120 (FIG. 1D), correlated signals are passed into the shift register 168 (FIG. 1E) during antenna revolutions 4, 8, l2 and 16. Since the upper two bits of the antenna revolutions counter 115 are disabled in the single frequency mode of operation, the AND gate 138 is enabled for every fourth revolution of the antenna and the designations of revolutions 8, l2 and 16 become without significance in single frequency operation.

Since the acceptance data by the shift register 168 and the erasing of data in the target data latch 142 are both initiated by the occurrence of a 2 antenna rotation pulse, the one-shot multivibrators 124 and 126 are incorporated into the FIG. 1 system to assure that resetting of the target data latch 142 occurs after the data has been received into the shift register 168. The oneshot multivibrator 124 has a time duration near 7 microseconds and thus delays the occurrence of the reset pulse for target data latch 142 by such time. The target data latch reset pulse is also made to be near 7 microseconds in length by the one-shot multivibrator 126.

In the all frequency mode of operation, the shift register 168 accumulates all the target data which is received during the third, seventh, eleventh and fifteenth revolutions of the antenna. Since the shift register 168 itself is only one antenna revolution in length, the data 

1. Radio receiver and display apparatus for indicating signal receptions from and relative bearing location of a plurality of remotely located radio frequency signal emitting transmitters and for providing audio signal reception from said remotely located transmitters, said apparatus comprising: substantially unidirectional antenna array having a continuously rotating field pattern capable of selectively receiving radio frequency signals in sequence from multiple directions of azimuth around the locus of said apparatus and capable of converting said received signals into antenna electrical signals; selectively tunable superhetrodyne circuitry means including plural radio frequency amplifier stages coupled with first signal processing means for converting said antenna electrical signals into a radio frequency carrier strength indicating signal and also coupled with second signal processing means for converting said antenna electrical signals into a detected modulation envelope signal; audio signal processing circuit means connected with said second signal processing means in said superhetrodyne circuitry means for generating from said detected modulation envelope signal an audible audio signal indicative of any audio frequency modulation imposed on the signal currently received from one of said remotely located transmitters; gated automatic gain control circuit means connected with said selectively tunable superhetrodyne circuitry means for generating and storing during preliminary revolutions of said unidirectional antenna array field pattern a D.C. gain controlling signal correlated with the magnitude of said carrier strength indicating signal existing during reception of selected radio frequency signals by said apparatus and for applying said D.C. gain controlling signal to at least one of said plural radio frequency amplifier stages as a gain decreasing signal; signal peak detecting means connected with said first signal processing means in said tunable superhetrodyne circuitry means for generating, during antenna field pattern revolutions other than said preliminary revolutions, binary pulse signals each timed in substantial coincidence with a peak value of said radio frequency carrier strength indicating signal, whereby each of said binary pulse signals identifies a maximum degree of alignment of said unidirectional antenna array field pattern with one of said remotely located transmitters; signal storage and display means connected with said signal peak detecting means for accumulating said binary pulse signals during at least one revolution of said antenna array field pattern and for displaying to an operator a spatially oriented visual pattern representing said accumulated pulse signals.
 2. Radio receiver and display apparatus as in claim 1 which also includes: electrically controllable tuning means for changing the operating frequency of said selectively tunable superhetrodyne circuitry means and electrical control circuit means for coordinating revolutions of said substantially unidirectional antenna field pattern with changes of operating frequency of said selectively tunable superhetrodyne circuitry means and for identifying in said gated automatic gain control means said preliminary revolutions of said unidirectional antenna array field pattern.
 3. Radio receiver and display apparatus as in claim 1 wherein said signal storage and display means also includes signal correlating means connected with said signal storage and display means and with said signal peak detecting means for presenting stored signals from said signal storage and display means in time coincidence with subsequent antenna field pattern revolution real-time binary pulse signals from said signal peak detecting means and for re-storing a correlated signal for each time a stored signal and a subsequent antenna fiEld pattern revolution real time binary pulse signal occur in time coincidence.
 4. Radio receiver and display apparatus as in claim 1 wherein said signal storage and display means includes digital memory means limited to storing the binary pulse signals generated during one revolution of said unidirectional antenna array field pattern.
 5. Radio receiver and display apparatus as in claim 1 wherein said apparatus includes the combination of a substantially unidirectional antenna array having a directivity pattern incorporating one major frontal response lobe and at least one weaker minor response lobe angularly displaced from said major frontal lobe along with adjustable threshold circuitry means in said signal peak detecting means for amplitude distinguishing signals received via said major frontal lobe from signals received via said angularly displaced minor lobe.
 6. Radio receiver and display apparatus as in claim 1 wherein said audio signal processing circuit means also includes fast automatic gain control circuit means for at least partially smoothing amplitude variations imposed upon the audio frequency component of signal received through said continuously rotating substantially unidirectional antenna array field pattern.
 7. Radio receiver and display apparatus as in claim 1 wherein said gated automatic gain control circuit means also includes electrical circuit means for limiting the rate of increase of said generated and stored D.C. gain controlling signal during at least an early portion of said preliminary revolutions of said continuously rotating substantially unidirectional antenna array, field pattern, whereby signal received from a nearby transmitter is precluded from generating an excessively large overshoot in said D.C. gain controlling signal during said preliminary revolutions of said unidirectional antenna array field pattern while said receiver apparatus is operating in a high gain condition.
 8. Radio receiver and display apparatus as in claim 1 wherein said first signal processing means in said selectively tunable superhetrodyne circuitry means also includes low pass electrical wave filter means for rejecting frequency components of said carrier strength indicating signal that are substantially higher in frequency than components generated when said continuously rotating substantially unidirectional antenna array field pattern sucessively aligns with two radio frequency signal emitting transmitters angularly separated by the minimum resolution capability of said continuously rotating substantially unidirectional antenna array field pattern.
 9. Radio receiver and display apparatus as in claim 8 wherein said electrical wave filter means includes means for attenuating frequency components generated by multibladed aircraft propelling device interference with the signal path between said remotely located radio frequency signal emitting transmitter and said continuously rotating substantially unidirectional antenna array.
 10. Radio receiver and display apparatus as in claim 1 wherein said continuously rotating substantially unidirectional antenna array and said selectively tunable superhetrodyne circuitry means are selected to operate in the ultra high frequency band of frequencies; and said gated automatic gain control circuit means includes means for generating during the first two of four consecutive revolutions of said antenna array field pattern a D.C. gain controlling signal amplitude correlated with the peak magnitude of said carrier strength indicating signal while receiving radio frequency signals on one frequency.
 11. Radio receiver and display apparatus as in claim 1 wherein said substantially unidirectional antenna array has a directivity pattern incorporating a major frontal response lobe having a beam width near 60* when measured at the 3db down points.
 12. Radio receiver and display apparatus as in claim 1 wherein said apparatus includes means for connecting said gated automatic gain control means with a sufficient number of said radio frequency amplifier stages to limit the signal variation at said signal peak detecting means of said apparatus to a value near 6db during an input signal variation of 100db.
 13. A method for providing an aircraft with both homing guidance information and audio frequency communication information from a remotely located source of radio frequency energy so that said aircraft may be guided thereto comprising the steps of: sequentially scanning with a directive antenna array the space surrounding said aircraft for signals originating at said remotely located source of radio frequency energy; thereby converting said signals into antenna electrical signals having a modulation component imposed by the scanning of said directive antenna array; extracting from said antenna electrical signal an analog electrical signal corresponding to the antenna modulation component of said antenna electrical signals, said extracting involving amplifying and detecting said antenna electrical signals in radio receiver apparatus; generating from said analog electrical signal a digital pulse signal timed substantially at the peak of said analog electrical signal, said act of generating including the acts of differentiating said analog electrical signal and initiating said digital pulse signal at the zero crossing point of the differentiated signal; displaying against a background representing the incrementally divided scanning field of said directive antenna and the vector heading of said aircraft, a visual pattern representing time elapsed between antenna scan start and the occurrence of said digital pulse, whereby the operator of said aircraft is presented with a visual indication of the relative bearing between his own course of travel and the course of travel leading to said source of radio frequency energy; whereby said analog electrical signal provides both homing guidance information and audio frequency communication information from said remotely located source of radio frequency energy by way of signal from said scanning directive antenna having a substantial average value incorporating the maximum response of said directive antenna array in lieu of the nulled or minimum response of said directive antenna array. 